The invention relates to the area of electronic reproduction technology, and relates to a method and a means for controlling the engraving device of an electronic engraving machine for engraving print forms, in particular of print cylinders, for rotogravure by means of an engraving tool as a cutting tool, and relates to a corresponding engraving device.
In the engraving of print cylinders in an electronic engraving machine, an engraving device with an engraving tool as a cutting tool moves in the axial direction on a rotating print cylinder, and the engraving tool, controlled by an engraving signal, cuts a series of recesses arranged in a rotogravure raster, which recesses are called cups in the following, into the jacket surface of the print cylinder. The engraving signal is formed from the superposition of an image signal, representing the tone values between xe2x80x9cblackxe2x80x9d and xe2x80x9cwhite,xe2x80x9d with a periodic raster signal, which, together with the relative speed between the print cylinder and the engraving device, determines the geometry of the rotogravure raster. While the periodic raster signal effects a vibrating piston movement of the engraving tool, the image signal controls the penetration depths of the engraving tool into the jacket surface of the print cylinder, and thereby the volumes of the engraved cups, in a manner corresponding to the tone values to be reproduced. In a printing machine, the cups engraved into the print cylinder are filled with more or less ink corresponding to their volumes, which ink is then transferred onto the print medium during the print process from the cups of the print cylinder.
In the engraving in particular of rastered color separations of a set of color plates, high tolerances must be maintained with respect to the positions of the engraved cups in the rotogravure raster and with respect to the shape and depth of the engraved cups. Deviations of position of the cups from the rotogravure raster lead to moire phenomena and color play in the combined printing of the rastered color separations. Deviations of the penetration depths or, respectively, engraving depths alter the cup volumes and thereby the quantities of ink stored in the cups. This results in disturbing tone value falsifications on the print medium.
From DE-A-23 36 089, an electromagnetic engraving device is known, i.e., an engraving device with an electromagnetic drive element for the engraving tool. The electromagnetic drive element consists of a stationary electromagnet that is charged with the engraving signal, in the air gap of which the armature of a rotating system moves. The rotating system consists of a shaft, the armature, a bearing for the shaft, and a damping means. One end of the shaft goes over into a resilient torsion rod that is clamped in a spatially fixed manner, while the other end of the shaft bears a lever to which the engraving tool is attached. By means of the magnetic field produced in the electromagnet, an electrical torque is exerted on the armature of the shaft, which is counteracted by the mechanical torque of the torsion rod. The electrical torque deflects the shaft from an idle position by an angle of rotation proportional to the engraving signal, and the torsion rod brings the shaft back into the idle position. By means of the rotational motion of the shaft, the engraving tool executes a stroke directed in the direction toward the jacket surface of a print cylinder, which stroke determines the penetration depth of the engraving tool into the print cylinder.
Because the electromagnetic engraving device represents a system capable of oscillation, the engraving tool, in particular given abrupt changes of the engraving signal at steep density transitions (contours), has a transient response that is subject to error, which is influenced by the rotational inertia and the degree of damping of the rotational system. An error-prone transient response of the engraving tool results in engraving errors on the print cylinder or, respectively, disturbing changes in tone value in the printing. Given an insufficient damping of the rotational system, disturbing multiple contours arise at density jumps, due to overshootings of the engraving tool. Given an excessively strong damping of the rotational system, the engraving tool can follow too slowly at steep density transitions, and the target engraving depth is reached only at a distance after the jump in density, whereby steep jumps in density are reproduced imprecisely.
Thus, disturbing engraving errors can occur in a conventional electromagnetic engraving device, because the transient reactions can be controlled only with difficulty. In addition, the temperature-dependent degree of attenuation of the rotational system can be stabilized only at great expense.
From EP-B-0 437 421, a method is known with which the transient response of an electromagnetic engraving device is improved by means of a specific electrical driving of the engraving device. For this purpose, the image signal is briefly intermediately stored in a memory stage, and is supplied to the engraving device in a manner delayed by the storage time. During the storage time, a correction signal is derived from the image signal that can be adjusted in its amplitude and in its effective duration, which correction signal is supplied to the engraving device in a chronologically rapid manner.
From U.S. Pat. No. 5,491,559, a magnetostrictive engraving device is known for the engraving of print cylinders, i.e., an engraving device with a magnetostrictive drive element for the engraving tool. The magnetostrictive drive element essentially comprises a cylindrical actuator made of a magnetostrictive material, to which the engraving tool is coupled. The actuator is surrounded by an annular auxiliary coil through which a direct current flows and by an annular driver coil through which an alternating current flows. The direct current produces a constant magnetic field in the auxiliary coil for the pre-magnetization of the actuator. By means of the pre-magnetization, the actuator is expanded into a pre-stressed position. The alternating current produces a dynamic magnetic field with changing direction in the driver coil, which is superimposed on the constant magnetic field, whereby the resulting magnetic field causes, according to the direction, a further expansion of the actuator into an operating position for engraving or a contraction of the actuator into an idle position. The drive circuit for the magnetostrictive engraving device consists essentially of a current generator for the production of the direct current for the auxiliary coil and a voltage/current transducer. The image signal, containing the engraving information, and an alternating voltage with constant frequency are supplied as a raster signal to the voltage/current transducer, which raster signal effects the oscillating piston motion of the engraving tool for the production of the rotogravure raster.
The object of the present invention is thus to improve a method and a means for controlling the engraving device for engraving print forms, in particular of print cylinders, for rotogravure by means of an engraving tool as a cutting tool, as well as improving an engraving device, in such a way that disturbing changes of operating parameters of the engraving device are compensated in order to achieve rapid and error-free engravings.
According to the present invention, a method is provided for driving an engraving device of an electronic engraving machine for engraving a print form. Items of engraving information representing tone values are stored as engraving data. The information for engraving of a sequence of cups in a main engraving direction into the print form with an engraving tool as a cutting tool are retrieved. The engraving data that have been read out are converted according to a first function into at least one engraving depth target value per cup. A control signal for an engraving tool drive system is activated at a beginning of the engraving of a cup so that the engraving tool executes an operating stroke from an idle position in a direction towards the print form, and after the engraving of the cup the tool is guided back into the idle position for the reset element. In the engraving of the cups, operational strokes of the engraving tool are continuously measured from the idle position. During the engraving of the cups, a distance between a jacket surface of the print form and the engraving tool is continuously measured in a region of the engraving tool. Engraving depth actual values are determined from differences between the operational strokes and the respective distance. The engraving depth target values are compared with the determined engraving depth actual values. The control signal is modified given equality of the engraving depth target values and the engraving depth actual values. For the engraving of the cups, a motion relative to the print form in a secondary engraving direction is executed with the engraving device. A system is also provided for performing the above-indicate method.
By means of the invention, in particular the disturbing time-dependent drift of a conventional electromagnetic engraving device due to the instability of the electronic driving and the damping is reduced. In addition, during the engraving different material hardnesses of the print cylinder and distance fluctuations between the engraving device and the print cylinder due to non-roundness or deformation of the print cylinder are compensated without the use of a conventional mechanical sliding foot, which normally provides for a constant distance between the engraving tool and the print cylinder. Overall, short engraving times and a good engraving quality are achieved.
The invention is explained in more detail below on the basis of FIGS. 1 to 4.